I. Field
The following description relates generally to wireless communications, and more particularly to reference signal design to support legacy user equipment in LTE A.
II. Background
Wireless communication systems are widely deployed to provide various types of communication; for instance, voice and/or data can be provided via such wireless communication systems. A typical wireless communication system, or network, can provide multiple users access to one or more shared resources (e.g., bandwidth, transmit power, . . . ). For instance, a system can use a variety of multiple access techniques such as Frequency Division Multiplexing (FDM), Time Division Multiplexing (TDM), Code Division Multiplexing (CDM), Orthogonal Frequency Division Multiplexing (OFDM), and others.
Generally, wireless multiple-access communication systems can simultaneously support communication for multiple mobile devices. Each mobile device can communicate with one or more base stations via transmissions on forward and reverse links. The forward link (or downlink) refers to the communication link from base stations to mobile devices, and the reverse link (or uplink) refers to the communication link from mobile devices to base stations.
Wireless communication systems oftentimes employ one or more base stations that provide a coverage area. A typical base station can transmit multiple data streams for broadcast, multicast and/or unicast services, wherein a data stream may be a stream of data that can be of independent reception interest to a mobile device. A mobile device within the coverage area of such base station can be employed to receive one, more than one, or all the data streams carried by the composite stream. Likewise, a mobile device can transmit data to the base station or another mobile device.
Having multiple transmit antennas in wireless systems helps in getting transmit diversity and/or higher data rate. Transmit diversity relates to the performance improvement obtained when a signal is sent multiple times over different transmit antennas. The key idea is that, when the channel gains from different transmit antennas are independent, the chances of the channel gain from different transmit antennas to the user equipments (UEs) being small simultaneously decreases exponentially as the number of transmit antennas is increased. Probability of failure in this case is approximately p^Nt where p is the probability of failure when only one transmit antenna is used and Nt is the number of transmit antennas used. On the other hand if the signal was sent from the same antenna multiple times, if the channel was bad in the first instance, it is likely to be bad for the remaining transmissions and hence the probability of failure continues to be equal to p (approximately).